1. Field of the Invention
The present invention relates to an image processing apparatus, and more particularly to techniques of composing image signals of a plurality of images in one picture.
2. Related Background Art
Multi-picture display is now frequently adopted by using a liquid crystal projector or a large display device such as a plasma display device. In the multi-picture display, images received from a DTV (digital television), images reproduced from a DVD (digital versatile disc), and images processed by a personal computer are used. It is thought that a large screen display device will become a main display device for multimedia in home use, and applications to integrally displaying images of various media on one display device has been desired.
In such a background, an image display system directed to applications mainly to DVD is required to have not only a function of displaying a plurality of images at the same time but also a function of composing a plurality of images into one multi-picture. Such images include an image from digital broadcasting, an image from a WWW browser, an image from an e-mail, an image from EPG (electronic program guide), a so-called GUI image such as a user support image of peripheral apparatus compatible with an IEEE1394 serial bus.
In order to realize such functions, a memory is necessary which is used for developing a plurality of input video images and GUI images. This memory has a structure constituted of a video image plane, a GUI image plane, and an alpha plane used for controlling the composition of video images and GUI images. In accordance with alpha values written in the alpha plane, a video image selection, a GUI image selection, and a composition of video and GUI images such as alpha blending can be processed on a bit basis.
In such a conventional image display system, if the memory for developing a plurality of input video and GUI images uses a dedicated alpha plane for controlling the composition of these images, the capacity of alpha plane is generally 8 bits per pixel multiplied by a display resolution.
For example, use of a progressive display device of a high resolution of 1080 P for HDTV of DTV field requires an alpha plane memory having a capacity of 1920×1080×8=2 MB. This increase in the memory capacity may cause an increased load of a CPU which develops alpha values to GUI images or of a graphic accelerator.
In order to solve this problem, a chroma key method has been proposed. With this method, without using an alpha plane, a plurality of input video and GUI images can be composed by inserting a specific key color in GUI image data.
The chroma key composition method relies upon that a CPU can control colors of a GUI image so as not to hit a key color. If data unable to be controlled by CPU is developed in a GUI image, a probability of a key color hit increases and an intended image composition may not be performed. Such the data unable to be controlled by CPU includes still image data derived from BS digital data broadcast, image data on the Internet, natural image data taken by a digital camera or the like.